Photo-electric tube



July 23, 1935.

A. R. OLPIN PHOTO ELECTRIC TUBE 8 Sheets-Sheet 1 Filed April 6, 1929July 23, 1935.

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July 23, 1935. A. R. OLPIN 2,008,874

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July 23, 1935. QLPIN I 2,008,874

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PHOTO ELECTRIC TUBE Fi1ed Apri l 6, 1929 8 Sheets-Sheet 7 lNVENTO/P A. R0L P/N' A T TUR/VEY Patented July 23, 1935 warren stares PATENT cariessnore-sarcasm TUBE Application April 6, 1929, Serial No. 353.1% 13Claims (or. 25(l-27.5)

This invention relates to optics and electrooptics and particularly tolight-sensitive electric devices and methods of making them.

An object of the invention isto increase the sensitivity ofphotoelectric tubes.

, to blue light and responds very little to red light.

Moreover, its sensitiveness even to blue light is so small that thephotoelectric current has to be given enormous amplification for mostpurposes.

In accordance with the present invention photoelectric tubes areprovided which have an increased maximum sensitivity which may be madeto be'many times that of tubes heretofore used and which are highlysensitive to red light.

" Moreover, it has been discovered that sensitiveness to red light maybe obtained in a tube employing light sensitive material which is notsensi- .tive to such. light by associating with the light sensitivematerial another substance which is capable of modulating the red lightwith a resonance frequency of its own to produce modulation frequenciessuch as a side frequency (sum or difference frequency), to which thephotoelectric material employed is highly sensitive. In

cident red rays therefore may produce a large photoelectric current evenwhen the photoelectric material employed is not at all sensitive to suchrays. Thus, for example, a layer of sodium may be employed as the lightsensitive substance and cc covered with a thin layer of dielectricmaterial obtained by heating flowers of sulphur (which may containtraces of water) in asidetube connected to the tube and exposing thesodium surface in vacuo to the substance. given off. There is thusformed upon the sodium surface a thin layer of dielectric which iscapable of modulating incident red light by a resonance frequency of itsown in the infra-red to produce a modulation frequency in the region towhich sodium is particularly sensitive. Such a tube may be furthergreatly improved by methods herein described.

Many materials suitable for use with a socalled light sensitive materialto-modulate incident light and so shift the effective frequency 5 to adesired position are mentioned in this specification. In accordance'withthe invention any of these materials or combinations of them or othermaterials having the same property may be used for modulating, not onlyinphotoelectric in tubes, but in other devices where a change offrequency may be utilized to advantage.

A more detailed description of the invention will now be given havingreference to the accompanying drawings.

Fig. 1 shows schematically a layout of the apparatus used in .makingphotoelectric tubes according to this invention.

Figs. 2 to 28 are 'curves showing characteristics of cells madeupaccording to this invention.

Referring to Fig. 1, the apparatus used in making photoelectric tubesaccording to this invention will now be described. The tube propercomprises a glass vessel 5 having a substantially spherical-shapedportion 6 which is about two inches in diameter. The cathode of the tubeis formed on the inner surface of the portion 6. An anode l in the formof a nickel ring is supported from'the stem 8. The vessel 5 is connectedto a vacuum pump station by a glass tube 9. Between the tube 9 and thevessel 5 is a distilling tube In to which is connected a side tube llwithin which photoelectric material within a glass capsule I2 is, placedprior to being distilled into the vessel 5. Atubulation I 3 comprising aU-shaped portion l4 and a sealed end-tube i5 is also sealed to the stem8 'of tube 5 through v which the various dielectric materials employedaccording to this invention are introduced. 40 Dewar flasks l6 and I!are provided forcooling the U-tube I4 and the dielectric tube l5,respectively.

' The vacuum pump station comprises a vacuum pump "3, a mercury vaporpump l9, an arrangement 20 for introducing inert gas, a McLeodpressuregauge 2|, a liquid air trap 22 and an ionizatign gauge23.

The cathode of one tube to bedescribed by way of example comprises anopaque layer of potassium, the surface of which is treated with thegaseous material evolved from flowers of sulphur when heated. A capsulel2 of previously purified potassium is inserted in the chamber H whichis then sealed. The flowers of sulphur are placed in the tube l5 whichis then sealed to the tubulation l3. The chamber II and the capsule l2are then heated until the potassium within the capsule I 2 issufficiently molten to break through the end crust of oxide and flowinto the bulb 24. The chamber II is then sealed off from the bulb 24.From the bulb 24 the potassium is successively distilled through thebulbs of the distilling tube In to form an opaque layer of potassium onthe inside of the spherical member 6. Prior to the formation of thislayer of potassium the system is thoroughly evacuated and the vessel 5outgassed by heating. By means of a point flame a window 25 about oneinch in diameter is made on one side of the bulb 6 through which tointroduce the exciting light. After the potassium coating has beenformed the side tube I5 is heated until the sulphur melts. Surprisinglylittle gas' is given olT during this heating, the manometer reading ongauge 2| being scarcely detectable. During the time that the sulphur isbeing heated, a test circuit comprising battery 26 and galvanometer 21is completed by the closing of switch 28 in order that any change inresponse tolight may be noted readily. The pump is left running duringthe treatment of the surface. The sulphur sublimes easily with littleadditional heating, so that it is easy to control the amount enteringthe photoelectric cell 5. Light from a constant source is directednormally through the window onto the back of the tube. The amount ofdielectric introduced is determined by the sensitivity conditionsdesired.

Table 1 gives a typical history of the process of sensitizing apotassium surface and the relative current values for two differentpolarizing voltages at every stage in the making.

Table 1 I. E. current at cathode voltages. Cathode history 8 Volts 50Volts Units Units Freshly distilled potassium 34 38 Very slight amountof sulphur sublimcd in tube; no change in surface color 58 62 Slightlymore sulphur vapor on surface 118 141 More sulphur vapor; no mlorutionol icc 179 217 More sulphur vapor; faint rose color appears M5 483Pumped l0 mimncs 815 Light passed through dillusing glass into cell 512542 Pumped another 10 minutes 905 4% Slightly more sulphur (surfacecolor changes.

to golden) 840 380 Sulphur tube sealed oil 846 418 Verylow pressure ofargon 1, 360 2, 280 Argon pressure increawd to .1 mm 1,470 4, 120

It will be noted that once the dielectric film begins to build up, asshown by a change in the surface color, the current values were largerat low voltages than at high. This has been quite generallyobserved'with this 'type' of tube, the voltage versus current curvesshowing a maximum sometimes at .cathode voltages as low as -5 volts.Whgn argon was admitted at low pressures, however, the gas curvesincreased from this maximum. A typical series of these voltagecurrentcurves for a surface treated with sulphur vapor is shown in Fig. 2.Since the light on a scale making the emissions under 9.0-

tion of total light equal. i

Table 2 Color of exciting light. E 5 23'" I gig KS tube K11 tube 1 WhiteNone 374 374 374. Violet #76 1] ll 20 Blue #78 93 101 132 Green. #60 5.)44 Yellow #10 72 9 Red. #20 58 11 1 The filters used and designated bythe symbol EK were standard'filters made by the Eastman Kodak Company.The color temperature of the constant light source used for this tablewas not known but Fig. 3 shows the relative sensitivities of KS and KHcells to white light of color temperature 2848" K. The new tubes arevery stable and show little change in emission upon aging, the tendencybeing rather to increase in sensitivity during the first few days aftersealing them In Fig. 4 are'plottecl some curves sensitivities throughoutthe ofi the pump. showing relative spectrum. These curves taken withspectrally resolved light are corrected so that the ordinatesrepresenting current per unit of light energy incident on the surface,are equal at the selective maxima.

It will b noted that the large selective maxima for these curves lie inthe same spectral region but that the maximum for this potassium sulphurcell is slightly displaced toward shorter wave lengths. The long end of.the curve for the cells can berepresented as an amplified senstitivitycurve for pure potassium plus an additional curve representing a newmaximum symmetrically drawn about 6000 A". The importance of this secondmaximum should not be overlooked. It falls in that portion of thespectrum where the energy content of radiations from most illuminatingsystems is large, and its presence figures strongly in increasingthe-response of a tube to light.

Tubes having a cathode comprising rubidium .treated with sulphur vaporin the manner hereinbefore described for tubes employing potassium havealso been made. Fig. 5 shows the spectral distribution relationshipbetween wave length and photoelectric current for this type ofphotoelectric tube. The selective peak appears at a slightly shorterwave length after treatment than before and no second maximum in the redI appears.

The voltage vs. current curve shown in Fig. 6 is unique in that thecurrent reaches'its maxi mum value with not more than minus three voltson the cathode rapidly decreasing to a constant value from about minustwenty volts up.

The admission of argon at low pressure into the tube, however, shows anamplified efiect at voltages less than the ionization potential of thegas as was also the case with potassium tubes. I

Tubes in which the photoelectric material is caesium were also made. Thecaesium, however, was in the form of a thin transparent film depositedon conducting metal coatings such as magnesium. The characteristics ofthis type of tube are shown in Fig. 7..

Tubes in which the photoelectric material is sodium have likewise beenmade. By following the same procedure with sodium as with potassium,tubes were made having much greater sensitivity than the potassiumsulphur tubes to light of color temperature 2848 K. Moreover,

the most pronounced increase in sensitivity was found to exist in thered end of the spectrum.

Fig. 8 shows a typical curve giving photoelectric current per unit ofexciting light energy for such a cell. The appearance of the maximum atwave length 3600 A may be due to some absorption of the incident lightby the glass walls of the tube. The peak, however, has not been shiftedappreciably by the treatment of the sodium layer with sulphur vapor. Theappearance of a new maximum at approximately 5000 A" is significant.

In Fig. 9 are shown three curves comparing the sodium tube with twotypes of potassium cells under identical conditions showing theirrelative sensitivies throughout the spectral region investigated.

The maximum at low voltages appeared in the voltage'vs. current curvesfor the sodium tubes also but at slightly higher voltages than forpotassium. To make certain that this was not surface charging a pointflame was applied momentarily near the edge of the tube window to spreada thin conducting film of sodium over any thoroughly non-conductingsurfaces. 'Ihe response of the tube to light under this condition wasabout double that of the surface without the film of sodium and oncemore the increase was chiefly in the red and infra red regionsas shownin Fig. 10. The new spectral emissivity curve can be broken up into theregular curve for sodium greatly amplified plus a greatly enhancedmaximum at wave length 5090 A". In Fig. 11 is shown a history of thespectral distribution of emissions for each step in the sensitizingprocess. 50

At this stage in the development of red sensitive sodium tubes anaccident was capitalized to produce a surface having the 'greatestresponse to,

light of any previously studied. This was due in part to another andforins the subject matter of an application of G. R. Stilwell, SerialNo.

356,095, filed April 18, 1929. This new photoelectric tube wasespecially sensitive tolong wave light, the greatly enhanced selectivemaximum in the spectral distribution curve broadening appreciably andthe long wave limit shifting out to at least 1 mu, as shown in Fig. 12.The accident referred to was the cracking of a tube leading to an almostcompletely made tube, letting in airat atmospheric pressure. Inrepairing the crack A the tube was re-evacuated. No photoelectric provethat. the resulting tube was not a freak many-other tubes into which airwas deliberately admitted, have been made and show the sameimprovement." I

It appeared thatgjjhe effect of air on the surface was to cause abroadening of the new selective maximum on the long wave side orpossibly 7 a shift of this maximum toward the red. Fig. 13 shows thatpure Na. surfaces can be sensitized to long wave-lengths by air andoxygen alone, but

comparison with Figs. 12 and 14 indicates the advantage of the presenceof sulphur vapor. In

comparing these curves, the wave shapes are.

. cell treated with sulphur vapor, a sodium tube similarly treated,another such sodium tube with a. thin film of sodium deposited .on topof the dielectric and finally a sodium tube treated with both sulphurvapor and air as described above. The emissions are in terms ofmicroamperes per lumen, and the color temperature of theexciting lightis 2848" K.

The stability of the tubes was apparent when a constant light was leftincident on the cathode surface for over an hour without so much asonehalf of one per cent change in current output.

The linearity of response to light was checked for both potassium andsodium surfaces sensitized by the methods described above over anextended range. The variations in light intensity were effected bymoving the lamp source along a photometer track. The measurements weremade on a Compton electrometer, using the steady deflection method.

.I'he words sulphur vapor have been used advisedly in the foregoingpresentation, for it was early discovered that the actual sublimation ofsulphur onto the surface was not essential. In fact. equally good tubeswere made with only the volatile gases liberated from sulphur onheating. These gases could be held in a liquid air trap between thesulphur and the tube, and then by lowering the liquid air flaskproperly, the amount of gas actually entering the coated tube could beaccurately determined.

A surprising observation was the very slight amount of gas actuallynecessary to produce the most sensitive tubes. Both the oil and mercuryvapor pumps were left connected to the tube and ally occluded orcontained in commercial flowers of sulphur is very small. Yet when atube was sealed off the pump station with a side arm containing sulphurstill attached, and acurve taken showing the spectral distribution ofresponse to light (Fig. 16) there was appreciable sensitivity out to 1Evidently some gas had been liberated from the sulphur, but the amountwas too small to be detected with a voltage-current curve or by anyknown methods.

The question as to the nature of the activating gas contained in sulphurwas a challenging .one since it was present in such small quantities. Itcertainly was not air or oxygen for these gasescould not be condensed atliquid air temperatures. The assumption that it was water vapor,hydrogen sulphide or sulphur dioxide seemed most natural therefore, andtests were made checking the effect of these gases.

l these tests.

A large number of potassium coated and sodium coated tubes weremade, thecoating of each being treated with some gas in question. The chemicalaction and resultant color of the surface were about the only thingsthat could be compared in the case of the various potassium tubes,

all of them having selective maxima at approximately the same wavelength, viz. i 4300 A, and the long wave limit not varying appreciablyfrom one tube to another with one notable exception.

' The potassium coating on which water vapor was admittedwas decidedlymore red sensitive than the others. However, the surface color wasdecidedly different from that of the sulphurvapor tubes and thesensitivity to unresolved light not so marked.

The gas causing efiects on potassium most nearly like those observedwhen using sulphur vapor was S02, the only one to give the bright goldencolor and the one producing the most sensitive surfaces to unresolvedlight. The sample used, together with the sample of H2S, was certifiedby the manufacturer to have a high degree of chemical purity;

In Fig. 17, data depicting the spectral distribution of electronemission are given for potassium tubes sensitized with sulphur vapor,sulphur dioxide, hydrogen sulphide, water vapor, oxygen on sulphurvapor, tellurium vapor, phosphorousvapor and iodine vapor.

Sodium surfaces offered the most significant foundation for experimentsin determining the composition of the activating gas, due to the markedvariations in the shape of the spectral distribution curves fordifferent dielectric films. Although SO'z had appeared as the importantconstituent of sulphur vaporin experiments on potassium, such was notthe case when sodium was the base metal. As shown in Fig. 18 thespectral distribution ofsensitivity after treatments with this gas wasquite different from the sulphur vapor curves. 'On the other hand watervapor produced curves very similar to them and the colors of the treatedsurfaces were quite identical, viz. a dull grey.

There seemed only one conclusion to draw from The activating gas givenoff from commercial flowers of sulphur must be a com bination of watervapor and sulphur dioxide, and the dielectric films formed must besodium and potassium bisulphites. This conclusion seemed the moreplausible when it was recalled that the sulphonic radical SO2.OH is animportant radical in many organic dyes and that photographic plates haveheretofore been sensitized to light,

There was no reason to suppose that sodium.

bisulphite was the only substance used in photography which could beapplied to the field of photoelectricity. Accordingly, other sensitizingdyes were introduced onto the light sensitive surfaces of alkali metals,and marked increases in photoelectric emission noted. In every case th'eamount of dye required was very small, as in plate sensitizing, yet' thecolors and hues appear-' lug-on thecathode surfaces especially with thedeposition of thin top films of sodium and potassium, were many andvaried.

Some of the tubes containing the dyes had to be immersed in liquid airflasks to prevent their breaking up-and passing over into the cell withthe action of the pump. Others had to be warmed before they sublimed,and in such cases it is not only possible but likely that partialchemical decomposition occurred. Nevertheless, the well known organicabsorption radicals for the visible region, as the methyl group CH3, thenitroxyl group N02, the amido group NHz, the bromine group, the methoxylgroup CHzO, the carboxyl group C0.0H and the sulphonic group SO2OH,probably were fairly stable.

The first dyes used contained the sulphonic radical and were not knownas photographic sensitizers. They were placed in a. side tube beyond theliquid air trap l4, and then heated after the alkali metal coating wasmade. Upon heating some gas passed through the liquid air trap into thetube and was pumped out. This was probably nitrogen, hydrogen orpossibly some hydro carbon compound, for no chemical action with thesodium or potassium occurred. The gas retained in the liquid air trapwas very effective in sensitizing the metallic surface of the tube whenallowed to enter in smallquantities. A very thin film of the alkalimetal deposited on the colored surface always enhanced the emission. InFig. 21 are found curves showing relative sensitivities in the visibleand infra-red spectral regions of sodium cells treated with the isomericcompound tro pa-eolin 000 No. 1 (HO.C10H6.N:N.CsH4.SO3Na) and sodiumindigo disulphonate. Both compared favorably with the sodium and sulphurvapor cells described hereinbefore. Curves for the correspondingpotassium coated tubes, using these dyes are given in Fig. 20.

To determine whether the sensitizing process was limited to sulphurcompounds, 2. tube was made with a rosaniline base [OH.C(C6H NH2)3] anda sodium coating' Although no sulphur was contained in this compound,the surface treated showed a good response to light throughout thevisible and near infra-red as shown in Fig. 22.

The remainder of the experiments using dyes have to do with theapplication of the sensitizing dyes used in photography. Because of itshistorical importance, having been first used to sensitize photographicplates to green and yellow light in 1882, eosin blue.

[CcH4 (cocennonw 20'] was tried first. Here was a compound containing nosulphur and no hydroxyl group, but possessing a liberal amount ofiodine. Although a decided increase insensitivity of the tubes waseffected through use of this dye '(Fig. 23) it was not so satisfactoryas many others especially as regards response to the red light..

v Unusual results in sensitizing photographic plates to red by usingalizarine blue Similar results attended the use of dicyanine.

the tubes made with addition of ammonium sulphite vapor being quite redsensitive. The exact chemical formula is unknown for this compound andother supposedly still better dyes-carrying the latter part of the sameword in their trade names, as Kryptocyanine and Neocyanine. Frominformation available on these substances, it appears that Kryptocyanineshould be better in the near infra-red than dicyanine which in turnshould be better than cyanine (CzsHasNzI). of these indications wereborne out on photoelectric tubes, the dicyanine causing a greaterelectron emission from alkali metal surfaces than cyanine, and verysmall amounts of Kryptocyanine producing one of the broadest selectivebands in the spectral distribution curve for sodium tubes so farobserved. This is shown in Fig. 2'7.

Most of these sensitizing dyes had a tendency to volatilizespontaneously in a vacuum and the tubes containing them had to beimmersed in liquid air during the later stages of evacuation of thecells. Then by lowering the liquid air slightly easily controllableamounts would pass onto the light sensitive surface. Whether or not thedyes suffered a chemical decomposition during this volatilizationprocess is not known, but it is possible that the more complicatedcompounds break up into simpler ones.

While there seemed to be quite marked correlation between the absorptionof the organic dyes and the photoelectric emission from alkali metalsurfaces on which they were deposited, there was no evident reason forthe enhanced sensitivity always appearing as a new selective maximum atthe same wave lengths. Certainly the position of the new maximum seemedto be characteristic of the base metal and not the dielectric on itssurface; perhaps any dielectric regardless. of its absorptive propertieswould be sufiicient to give the increased sensitivity to red light. Totest this various colorless dielectrics and dielectrics practicallytransparent to visible light were substituted for the dyes. Most ofthese were liquids but could easily be held in side tubes with liquidair. The technique involved in their use was exactly identical to. thatfor the dyes...

Acetone (CH3.CO.CH3), acetic acid (C2H4o2) carbon bisulphide (CS2),methyl alcohol (CI-LOH) carbon tetrachloride (CCli), benzene (CsHs),chloroform (CHCls) phenyl mustard oil (CsH5-N-C=S) nitrobenzene(C6H5.NO2) and water-vapor (H2O) were used. Benzene and water vaporwe're efiective in making the best photoelectric tubes, the redsensitivity of sodium surfaces properly treated with them beingespecially marked, as shown in Fig. 28. There was absolutely no evidenceof chemical action when benzene was admitted to the tube, even though athin film of sodium was deposited on top of it. In" fact, the surface ofthe completed photoelectric tube looked exactly like that of puremetallic sodium. This, of course, is not surprising since the alkalimetals are preserved in benzene. In contrast to the behavior of benzeneon sodium was that of water-vapor, whose powerful affinity for thealkali metals necessitates their preservation in benzene, oil or sealed,air-tight containers. Yet both benzene and water-vapor have similarproperties as regardsincreasing photoelectron emissions from sodium andpotassium surfaces. 1

In every case mentioned in the preceding para graph, with the exceptionof that in which carbon tetrachloride was the dielectric used, there wasan appreciable amount of red-sensitivity developed. Moreover, thespectral distribution curves show evidence of a new peak in thephotoelectric emission curve near x5000 A for sodium and a much lesspronounced peak at A6000 A for potassium, these accounting for theincrease in response to red light. In the case in which carbontetrachloride was used no deflection of the galvanometer was observedwhen the exciting light was passed through a red filter, cutting out allwave lengths under 6000 A.

When working with carbon bisulphide (CS2) and nitrobenzene (CsHsNUz) thepreviously described peak in the voltage-current curve at low voltageswas observed in very much greater prominence. At one time during thetreatment of sodium with CS2 the current output with -15 voltspolarizing voltage was four to five times its value when the polarizingvoltage was increased to 100 volts.

Although the invention in the specific aspects heretofore described isindependent of any theory which may be advanced to account for theresults obtained, the accumulated evidence points so. strongly to thetheory about to be stated that it is believed to be correct, at least"in its main aspects. This theory may be briefly stated as follows:Shifts in the sensitivity curves of cells employing the variousmaterials with which the photoelectric surfaces are treated are producedThis side frequency, or frequencies, (which may be a sum or a differencefrequency, or both, with respect to the incident frequency and theresonant modulating frequency) acts. more efiectively upon thephotoelectric material than the incident frequency per se. This efiectappears to be enhanced by employing a coating of this kind which isdipolar in nature. This may be due to the fact that the dipoles alignthemselves in definite geometric configurations such that there is lesscontinual impedance to the vibrations of the molecules in the higherenergy states.

The following is a discussion of the available evidence which points tothis theory. The curves shown inthe accompanying drawings may or may notbe truly representative-of the propthe spectrum. Many minor' questionswill be treated as the discussion proceeds.

- In the first place a survey ofthematerials used was made to determinewhether or not there existed some common constituent or element in thesubstances used, some activating agent common to all the dielectricsintroduced on the alkali metal surfaces. Certainly, the possibility -ofthere being traces of water vapor had to be acknowledged. Every eifortwas made in many cases to eliminate this condition and there was notenough present in the evacuated tubes at the time the metal coatingswere made to be noticeable. But, as pointed out previously, thesubstances which were evaporated or sublimed onto these coatings may nothave been water free. In fact, admission of admittedly moist air ontothe surfaces of the sodium coatings with proper subsequent treatmentgreatly enhanced the emission. Moreover, the bulky, complex organic dyecompounds usually were found in very fine crystalline granules, nodoubt, containing water of crystallization in no small quantities, andsometimes decomposing with the actual formation of water vapor. Also,although the samples of transparent liquid dielectrics were obtainedfrom a reliable chemical house as certified chemically pure material,there is no assurance that they were totally water free. Finally, glassblowing and vacuum pump technique are of such a nature that when aliquid frozen in liquid air is sealed onto an evacuating system atatmospheric pressure, it is impossible to entirely eliminate all tracesof water from the system.

Although the amount of water vapor which could always have been presentwith the dielectric was admittedly small, it may not have beennegligible, on the other hand, many of the experiments showed itspresence to be an asse The great majority of the dielectrics success-'fully used in these experiments were dipolar substances, or at leastbroke up into groups or radicals which were permanent electricaldipoles. Dipolar substances are substances, the two ends of whose-molecules carry permanent electrical charges of opposite signs. Oxygenwas an exception but it probably did not exist by itself on the surface,forming NazO or K20 immediately. In fact, in every experiment'performedexcept the one with benzene there appeared to be some chemical action atthe surface. Now dipoles naturally are favorable to association, aspointed out by Gerlach and others, and this regular association isnothing more nor less than a preliminary stage of microcrystallinecharacter. It is even conjectured that the very symmetrical,ringcompound, benzene, may manifest such a character upon solidifying inthin films. .Now as this microcrystalline surface structure built up,water vapor present may have been taken up as water of crystallization.Such a condition was even intimated in the case of sulphur vapor, forthe hydrated mono-sulphides of sodium and potassium, as

Nazis-91120 01 K2S.5H20,'

sodium trisulphide NazSsBHzO and the corresponding bisulphideNazS2.3H2O, are golden yellow in color, as were the surfaces of thetreated photoelectric tubes. Moreover, this water of crystallizationplays a definite role in the crystal structure contributing to thevibration spectra and dcporting itself generally as an actualconstituent. By the very nature of the existence of dipoles as pairs ofelectrical charges, it is likely that they oriented themselves on thesurface in a uniform manner, such as Langmuir found for drops of oil onliquid surfaces. Moreover, the olicntation would undoubtedly be one suchthat the eddy fields they set up at the surface would tend to opposethat due to the polarizing voltage applied. Being nearly volatile theycould easily be oriented by applying a potential across the electrodes.The result would be the presence of, positive charges on the alkalimetal surface and negative charges slightly off the surface.

Obviously, the effect of such a surface configuration would betwofold: 1) the strong fields at the surface due to the presence of anelectropositive charge would greatly assist in pulling electrons fromthe metal surface, and (2) the eddy fields of the dipoles undoubtedlywould direct the electrons normally outward, where the field of theapplied voltage would readily accentuate this normality in tubesgeometrically designed as shown. There being no anode directly the pathof the light and no polarity at the window, the photoelectrons wouldcontinue until stopped byjthe glass of the window on which undoubtedlywould accumulate a negative charge. On the other hand, when the lightwas incident on the side of the tube at steep incidence, the anode wasdirectly above the illuminated area and practically all electrons wouldbe attracted to it.

The fact that the irregular hump in the voltage-current curves is moreaccentuated and at lower voltages for excitation with red light suggeststhat low velocity electrons were attracted to the anode more readily. Itwas only when the electrons were accelerated by greatly increasing thepolarizing voltage that these begin to shoot past the nickel ring. Inthe light of this, it appears that the rather broad maxima-in the curvesfor sodium, and their appearance at higher voltages than for potassiumand rubidium, was due to the more uniformdistribution of the velocitiesand especially the preponderance of electrons liberated by quanta of redlight, as shown by spectral distribution curves. The same explanationholds for the difference in the shapes of the curves for potassium andrubidium.

If the dielectric film were allowed to build up until the metal wasentirely coated, the continuous array of negative charges very likelyimpcded the emission of electrons. However, if at this stage a thin filmof light-sensitive metal was deposited on the surface, the negativecharges might well have acted as tiny grids tending to push theelectrons from the illuminated film, the direction of the force beingnormal to the surface, also. dipoles could function to simultaneouslypull electrons from photo-sensitive surfaces beneath it and pushelectrons from similar films above it.

A surface structure constructed as described has striking lightabsorption properties, being highly selective to different. bands of thevisible films deposited. By films or layers is not meant sharplydifferentiated strata. In all probability, the top film of alkali metalwas formed as atoms filtered down into intermolecular space, existingThus, it is seen how the layer of spectrum depending on thicknesses ofthe various in the dielectricso that the film may have been electricallyconnected to the base by a multitude of conducting atomic chains. Inthis respect the molecular dipoles may be considered as carefullyplanted absorption units in the light sensitive cathode surface.

That increased absorption of light was in great measure the cause ofincreased electron emission seemed the more likely since the presence ofoxygen andsulphur seemed always to improve the response tolight oflonger wavelengths and hydrogen alone had little effect in this region.The vibration energy of a molecule of the latter element is sufllcientlygreat to correspond to ultraviolet frequencies, whereas oxygen and.sulphur in particular have vibration frequencies corresponding tocolored light. The substitution of trum, if one or more of the hydrogenatoms is replaced by color groups as-NOz, OH, NI-Iz, SOaH etc., markedselective absorption in the visible region results. Thus the organicdyes absorb well bands of visible light because the simple ring compoundbenzene is a unique and distinguished chemical individual with specificcapacity for absorption.

A study of the selective absorption bands of the dielectricssuccessfully used in making photoelectric tubes as herein reported,however, showed no convincing correlation with photoelectron emission.For; lnstance, take the cases of cyanine and water vapor. Even thoughcyanin'e has a strong absorption band between i 4500 and 6500 A andwater vapor is transparent at these wavelengths, the photoelectricemission for sodium tubes treated with water vapor was greater over thiswavelength region than for sodium tubes treated with cyani ne. Similaranomalies were found for many other substances, benzene being 'astriking example. The tubes sensitized by means of this opticallytransparent compound manifest their best response to light atapproxiwith visible absorption bands, the facts certainly pointedstrongly toward an optical explanation. Investigation of the infra-redspectra of the materials used in sensitizing the tubes showed .asurprisingly close correlation between absorption bands there andelectron emission under the action of yellow and red light. 'Watervapor, the sulphur compounds, benzene and organic dyes generally showedpronounced similarity in their near infra-red spectra. These spectra,the vi-'-.

bration-rotation spectra, resolve'into lines and more or less sharpbands, and are due to the vibration of positive and negative chargewithin the molecule. It is only natural therefore that such vibrationspectra should be quite characteristic of dipoles.

The materials used exhibit characteristic spectra between wavelengths7000 and 20000 A. Now from the data plotted as wavelength vs. current itwas at once apparent that the new photoelectric maximumwhich appeared inthe spectral distribution curve ,for sodium was separated from thenormal selective maximum by a frequency diiference corresponding to awavelength of approximately i In other-words, if the frequency of lightcorresponding to the new maximum were added to the characteristicvibration frequency of the molecules on the cathode surface, thesummation frequency would be that corresponding to the selectivefrequency of pure sodium. I

It appeared then that the incident light was modulated as it wasabsorbed by the dielectrics on the cathode surface, quanta having toolittle energy to liberate electrons actually acquiring more from themolecules of dielectric. As long, therefore, as the sum of the incidentlight frequency and the vibration-rotation frequency of the cathodematerials gives a new frequency well within the limits required for theemission of elect'ronsfrom the pure metal currentswould be obtained.Comparison of the spectral distribution curves for pure sodium and forthe sodium tubes sensitized with dielectric films (Fig. 11) on thesurface showed that not only is the new peak or humps in the yellowregion for the latter case separated from the original selective maximumby a wave length corresponding'to a frequency of approximately 1,1, butthedifference in the long wave limits for the two cases corresponded toa similar separation. Moreover, these same relations held in a certainmeasure for all red sensitive cells regardless of the dielectric used'in the sensitizing process. Likewise the same observations carried overto the potassium cells-although the magnitude of the response to redlight was less.

Now if the light was actually modulated so that this, summationfrequency or upper side band appeared, thedifference frequency or lowerside.

band wouldbe certain likewise to appear. In fact, it seemed more logicalto conceive of the quantum yielding part of its energy to the absorbingmolecule than acquiring additional energy from it. Theoretical curveswere plotted for the ultra-violet, visible and near infra red and showthat the original curves of one selective maximum are changed to curveshavingthree maxima and that the humps in the visible agree with theexperimental curves of the accompanying drawings.

Since in the visible region theeife'ctive change is produced by theincident light acquiringenergy from the absorbing molecule, it is likelythat many of them existin the higher energy state. It appears,therefore, that the vibration-rotation frequencies characteristic ofdi-poles may be far less damped when existing, as it is surmised theydo, in the micro-crystalline filmson the surface.

Throughout this application the convention has been followed, insymbolically indicating the nature of the cathodes, of using thechemical symbols for designating the materials used, the symbol for thebase layer of photoelectric material appearing first.

I cw

The following materials have been used in making tubes according to thisinvention:

KSH

KHS

KNSH

KTe

KTl

KSe

NaSe

KSO2(H2O) TiNaS KSN NaSe (Air) KSO K (Indigo Disulphonate) NiKS Air NaEosin NaNHaSO3 NaS plus NHzSOs KNHaSOs KS plus NHaSOs Na dicyanine Nadicyanine plus NHsSOs K dicyanine K dicyanine plus NHsSOs KH on copperoxide Rb (Alizarine blue) plus NHsSOa K (Kryptocyanine) Ag KryptocyanineNa (Kryptocyanine) Na cyanine K cyanine K nitrobenzene plus sulphur Nanitrobenzene plus air Na plus CS2 on Pt Na plus spirits of turpentine Naplus acid benzene sulphonic- Na plus neocyanine Na plus neocyanine plusH2O.

It may be that the modulation referred to in this specification is one'wherein the energy of the side frequencies manifests itself only in theform of the emitted electrons and does not appear as light of new wavelength. The term side frequency as herein used is therefore not intendedto be limited in this respect to the case where light of new wavelengths is produced.

The term"light as herein used is intended to cover not only light withinthe range of the so-called visible spectrum but also electromagneticradiations both above and below that range.

The term equi-energy curve is' meant to mean a curve wherein theordinate refers to photoelectric emission per unit of incident light ofa particular wave length. In the accompanying; drawings the curvesplotted between wave length and photoelectric current per unit ofimpressed energy are equi-energy curves.

The term alkali metals as herein used is intended to cover the alkalineearth metals.

What is claimed is:

1. A photoelectric tube having an anode and a cathode comprising aphotoelectric substance sensitive to visible light and a light receivinglayer of dielectric material in contact therewith formed by exposingsaid substance to the action of sulphur vapor;

2. A photoelectric tube having an anode and. a cathode comprising aphotoelectric substance sensitive to visible light and a light receivinglayer of dielectric material in contact therewith formed by exposingsaid substance to the action of sulphur vapor and water vapor.

3. Aphotoelectric tube having an anode and a cathode comprising a layerof light sensitive material in contact with a layer of dielectricproduced by vaporizing an organic dye and bringing the vapor intocontact with said light sensitive layer. I

4. The process of making a cathode for a photoelectric tube having ananode and a cathode which comprises producing said cathode by forming invacuo a layer of photoelectric material sensitive to visible light andexposing the surface of said layer to a gas which produces thereupon alayer of dielectric other than a hydride of said photoelectric material.

- 5. The process of making a cathode for a photoelectric tube having ananode and a cathode comprising producing said cathode by-forming invacuo a layer of photoelectric material sensitive to visible light andexposing the surface of said layer to sulphur vapor. v

6. A photoelectric tube having an anode and a cathode comprising a-layerof alkali metalin contact with a layer of dielectric produced byvaporizing an organic dye and bringing the vapor into contact with saidlayer of alkali metal.

7. A photoelectric tube having an anode and a cathode comprising arelativelythin layer of solid alkali metal in contact with a layer ofdielectric produced by exposing the surface of said 4 layer of alkalimetal to the vapor of a colorless dielectric which is liquid at ordinaryroom temperatures.

8. A photoelectric tube having an anode and a cathode comprising a layerof alkali metal in contact with a layer of dielectric produced byexposing the surface of said layer of alkali metal to the vapor of,alizarine blue.

9. The process of making a cathode for a photoelectric tube having ananode and a cathode comprising producing said cathode by forming a layerof alkali metal in vacuo, and exposing the surface of said layer to thevapor of an organic dye.

10. The process of making a cathode for a photoelectric tube having ananode and a cath ode comprising'producing said cathode 'by forming alayer of alkali metal in vacuo, and exposing the surface of said layerto the vapor of alizarine blue.

11. A photoelectric tubehaving an anode and a cathode comprising aphotoelectric substance of the alkali metal group and a light receivinglayer of dielectric material in contact therewith formed by exposingsaid substance to the action of sulphur vapor.

12. A photoelectric tube having an anode and a' cathode comprising aphotoelectric substance of the alkali metal group and a light receivinglayer of dielectric material in contacttherewith

